Structure and Replication of DNA

Objectives Outline DNA nucleotide structure in terms of sugar (deoxyribose), base, and phosphate – State the names of the four bases in DNA – Outline how DNA nucleotides are linked together by covalent bonds into a single strand – Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds – Draw and label a simple diagram of the molecular structure of DNA

DNA structure DNA: a polymer of nucleotides consisting of: a nitrogenous base, deoxyribose sugar, (note C2- no oxygen) a phosphate group.

DNA structure Four bases are letters of the DNA alphabet: (A) adenine, (T) thymine, (G) guanine, ( C) cytosine

DNA structure Four bases are letters of the DNA alphabet: Amounts vary from species to species in characteristic, but not necessarily equal, ratios. Human DNA is 30.9% A, 29.4% T, 19.9% G & 19.8% C. (Eukaryotic chromosomes have poly-A tails important in replication, so there is more A.) Chargaff's rule (remember): %T = %A %G = %C

DNA Structure DNA nucleotides are linked together by covalent bonds into a single strand. What is a covalent bond? Electrons are shared.

DNA structure DNA nucleotides are linked together by covalent bonds into a single strand.

DNA structure A DNA double helix is formed using complementary base pairing & hydrogen bonds. A can only bind to T; form 2 H bonds G can only bind to C; form 3 H bonds

DNA Structure A simple diagram of the molecular structure of DNA: Note the two strands run in opposite directions (antiparallel).

DNA Structure In eukaryotic chromosomes DNA is wrapped around protein. Like thread wrapped around a bobbin to save space. DNA/protein subunits are called nucleosomes. DNA wrapped around histones produce subunits called nucleosomes

Structure and Replication of DNA

Objectives – Explain DNA replication in terms of unwinding the double helix and separation of the strands by helicase, followed by formation of the new complementary strands by DNA polymerase – Explain the significance of complementary base pairing in the conservation of the base sequence of DNA – State that DNA replication is semiconservative.

DNA Replication Base pairing leads to complementary strands. Because A binds to only T, and C to G, each strand acts as a template when separated. The order of bases on one strand can be used to add in complementary bases, duplicating the pairs of bases exactly.

DNA Replication Semi-conservative replication: when the double helix replicates, each of the daughter molecules will have one old strand and one newly made strand.

DNA Replication DNA Replication: requires more than a dozen enzymes. Replication begins at origins of replication. In bacteria, enzymes separate strands at one specific sequence of nucleotides. A replication "bubble“ forms; replication pro-ceeds in both directions at replication forks.

DNA Replication In eukaryotes, there are 100s or 1000s of origin sites per chromosome. Replication bubbles form, elongate, & fuse.

DNA Replication To begin: A pre-replication complex of proteins forms at the origin, including the enzyme helicase. Note

DNA Replication Helicase unwinds and separates the template strands at the replication fork. Proteins keep chains unwound.

DNA Replication DNA polymerase catalyzes elongation of new DNA at the replication fork.

DNA Replication DNA polymerase adds nucleotides at a rate of 500/sec. in bacteria and 50/sec. in human cells. Nucleotides float within the nucleus.

DNA Replication The polymerization of a nucleotide to the new strand is driven by the exergonic hydrolysis of pyrophosphate. (Ex: ATP splits off P-P)

DNA Replication DNA polymerases can only add nucleotides to the free 3' end of a growing DNA strand. It elongates in the 5' → 3' direction. Problem at replication fork: only 1 parental strand (3'→5' can be used as a template continuously to make the new leading strand that grows 5'→3'. The lagging strand grows in stages.

DNA Replication DNA elongates in the 5' → 3' direction. At replication forks: only 1 parental strand (3'→5') can be used as a template continuously, to make the new leading strand that grows 5'→3'. The 5'→3' parental strand, which makes the new lagging strand, is copied in stages. The stages are called Okazaki fragments and must later be spliced together by ligase.